Environmental demands have forced the pulping industry to develop improved cooking and bleaching methods in order to lower the lignin content of the pulp that enters the bleaching stages. This is both environmentally and economically advantageous. Spent liquor from the pulping stages are nowadays often efficiently recovered and recirculated. However, the effluent from the bleaching stages cannot be recovered in the same efficient manner so that the bleaching effluent may still have a negative impact on the environment.
It is, therefore, important to minimize the amount of lignin that has to be removed in the bleaching stages. The delignification in a kraft cook may be considered as a three-stage process; an initial phase involving the rapid removal of about 20% of the lignin; a slower bulk delignification stage; and, finally, an even slower residual delignification stage. The non-selectivity of the delignification, i.e., the decrease in pulp strength and pulp yield that accompany the lignin removal during the residual phase is often one important factor that limits the extent to which the lignin can be removed during the cooking stage in the production of bleached kraft pulp.
In conventional cooking processes, the digester is filled with wood chips and all the cooking liquor, which in the kraft process is an aqueous solution of sodium hydroxide and sodium sulfide, is charged at the beginning of the cooking process. In such processes, the initial alkali concentration is very high and because of the fast consumption of alkali, the concentration decreases rapidly to about one third after about one third of the total cooking time.
The basic understanding of the process kinetics of kraft pulping have suggested that the alkali concentration should be levelled out, i.e., decreased in the beginning of the cook and increased at the end of the cook so that virtually the same alkali level is kept through the cook to improve the selectivity, e.g., to increase the pulp strength at a given lignin content. This understanding has guided the development of many new modified kraft pulping processes.
One recent breakthrough within the field of pulp cooking is ITC.TM., which was developed in 1992-1993. ITC.TM. is described in WO-9411566, and shows that very good results regarding the pulp quality can be achieved by using the ITC.TM. process. This process is based mainly on the idea of using almost the same relatively low temperature in all cooking zones in combination with moderate alkali levels. The ITC.TM.-concept does not merely relate to the equalization of temperatures between different cooking zones, but a considerable contribution of the ITC.TM.-concept relates to enabling an equalized alkali profile also in the lower part of the counter-current cooking zone.
In many conventional systems, the white liquor charge is often split between the impregnation and the cooking stages, so called modified cooking, or the cooking stage is carried out in two parts so that the first part is concurrent and the second part is counter-current so that the alkali charge is divided between the impregnation, concurrent and the counter-current stages in order to keep the sodium hydroxide concentration at the same level. This practice is called modified continuous cooking (MCC). Further improvement may be gained by increasing the temperature in the washing section and adding white liquor to the wash liquid, so called extended modified cooking (EMCC). If the temperature is kept constant in all cooking stages, the process is called isothermal cooking (ITC), as described above.
The implementation of the present invention is possible in production facilities using all of the above mentioned cooking methods and other suitable cooking methods. An important feature of the present invention is that the effective alkali concentration should be substantially increased at the end of the cooking process. Unless otherwise specified, all effective alkali concentrations are measured as NaOH.
The fraction of lignin that is dissolved is surprisingly influenced to a large extent by the hydroxide ion concentration during the cooking process. However, the temperature does not seem to have much effect on the amount of residual lignin. Another important feature of the present invention is that only the hydroxide ion concentration and the temperature seem to have a significant influence on the rate of residual delignification. Hydrosulphide ion concentration only seems to have a marginal or no effect on the rate of residual delignification.
It is to be understood that somewhat different arrangements may be used depending upon, among other things, if an improved pulp production, a decreased lignin content in the pulp or an increased pulp yield or pulp strength is desired.
The method and device of the present invention provides for even better cooking characteristics than the above-described methods. According to the present invention, the cooking process is carried out at a relatively low and equalized alkali concentration to cause the residual lignin content to be relatively high while leaving the valuable cellulose and hemicellulose parts of the raw material mostly intact. The low alkalinity liquor, i.e. the spent liquor, in the digester is replaced with a high alkalinity liquor, i.e. a white liquor, at the end of the cooking process. This results in that the remaining lignin may be removed at a rate equal to that of the more selective bulk delignification phase. This high removal rate makes it possible to substantially increase the total overall selectivity of the kraft cooking stage, e.g., increased pulp yield and pulp strength at a given lignin content.
The novel method and device of the present invention enables the production of pulp that has a high quality and a very good bleachability. This means that bleach chemicals and methods can be chosen with a wider variety than before for achieving the desired quality targets related to brightness, yield, tear strength, viscosity, etc., of the final bleached pulp product.